Plant growth regulators (PGR's) affect the physiology of plant growth and influence the natural rhythm of a plant. More specifically, plant growth regulators may, for example, reduce plant height, stimulate seed germination, induce flowering, darken leaf coloring, minimize lodging of cereals, slow grass growth on lawns, reduce boll rot and improve boll retention in cotton.
Crop plants generally display a determinate or indeterminate growth pattern. Determinate plants have a defined period of vegetative growth followed by a defined interval of reproductive growth in which there is a maximum number of flowers initiated per plant. An indeterminate plant growth pattern, on the other hand, is characterized by an initial period of vegetative growth followed by a period where both vegetative and reproductive growth occur together. The length of the second period and the number of flowers produced are determined largely by the growing conditions.
Indeterminate crop plants continue to produce reproductive organs long past the point in the growing season where there is sufficient time for mature, harvestable fruit to be obtained from these organs. Thus, after a certain point in the growing season, further reproductive growth will have no impact on marketable yield. Because flowers and young fruit are strong sinks for carbohydrates and nitrogen, it is likely that the demand of these strong sinks causes a reduction in the available carbohydrates and nitrogen for the continued maturation of fruits that are far enough along in development to make a positive contribution to marketable yield.
Young, expanding leaves are also strong nutrient sinks. As a leaf approaches physiological maturity, it changes from a carbohydrate sink to a carbohydrate source due to the combination of increased photosynthetic capacity within the leaf as well as a decline in, and eventually a cessation of, growth. For indeterminate plants, leaves initiated after a certain point in the growing season will lack sufficient time to make the transition from carbohydrate sink to carbohydrate source. Thus, young expanding leaves drain the finite nutrient and carbohydrate resources of the plant that might otherwise be better allocated to the growth and maturation of marketable fruit.
One example of an important crop plant that displays an indeterminate growth pattern is cotton (Gossypium hirsutum). Cotton is a perennial of tropical origin that is cultivated as an annual in agricultural production in temperate and subtropical regions of the world. After an initial period of vegetative growth, a cotton plant initiates reproductive growth while vegetative growth continues. Flower buds (squares) appear, develop into flowers, and after pollination, give rise to fruit that are referred to as bolls.
Due to its indeterminate nature, squares continue to appear long after there is sufficient time left in the growing season for these squares to develop into mature, marketable bolls. The growth and development of these squares and young bolls drains the limited resources of the plant that might better be utilized by bolls that have sufficient time to mature to a marketable stage. Likewise, the shoot of the cotton plant continues to grow and initiate new leaves throughout most of the growing season. Many of the leaves that are initiated late in the growing season will never become carbohydrate source leaves due to insufficient time remaining in the growing season. Thus, these young leaves demand a portion of the carbohydrates and other nutrients that could be better utilized by the bolls that are likely to be harvested.
The peanut plant (Arachis hypogaea), a member of the Fabaceae family, is another important crop plant which exhibits an indeterminate flowering pattern similar to cotton. Like cotton, peanut plants will produce flowers so late in the season that these late season flowers will not have the time to develop into a marketable fruit before the first frost.
Tobacco (Nicotiana tabacum L.) production requires the early removal of the terminal bud by a process called "topping". Removal of the terminal bud stimulates growth and development of the axillary buds (suckers) Suckers are of no economic value and, if allowed to develop, would decrease the leaf yield. Chemical control of suckers is practiced through the United States typically using maleic hydrazide.
If an early-termination, plant-management strategy could be developed that reduces the number of late-season squares, flowers and young fruit, as well as eliminate further growth of the terminal portion of the shoot, then the carbohydrates and other nutrients would be reallocated to the strongest sinks remaining--namely, the young fruit and/or leaves. For cotton plants, as an example, the benefits of this reallocation of resources is likely to include some, or all, of the following: increased yield (due to larger bolls), increases in fiber quality, an acceleration of boll opening, a more complete defoliation and the ability to harvest earlier. All of these benefits can provide cotton producers with a significant economic advantage.
For an early termination strategy to be successful, inhibition of both vegetative and reproductive growth would have to occur without substantial injury to organs, such as leaves, stems, roots and fruit. A herbicide treatment could be used to terminate growth, however, the injury to the crop plant would be severe and likely cause significant reductions in yield.
Early growth termination could also have benefits in terms of late season pest management. The recent introduction of transgenic, insect-resistant cotton cultivars that have been genetically modified to express the Bacillus thurigensis insect toxin (BT toxin) has reduced the necessity and/or frequency of some insecticide applications. However, so-called insect-resistant "BT-cotton" is not the panacea for insect management in cotton production. In this regard, the insect resistance of BT-cotton is limited to a subset of the major insect pests of cotton. In addition, the resistance provided by the presence of BT toxin in crop plants is not permanent. Unfortunately, there is evidence that some individual members of pests species otherwise susceptible to BT toxins may no longer be affected by the BT toxin.
Thus, insecticides remain a vital component of cotton production systems along with BT cotton. Because insects that are either resistant to currently known insecticides and/or tolerant to the BT toxin will increase over time, new insect management strategies need to be developed to ensure that the cotton production levels meet future demand.
As a means to both lower production costs and mitigate the development of insects with insecticide resistance, the elimination of late-season insecticide applications is currently being recommended by some cotton production experts. The basis of this recommendation is that after a certain point in the season, insect damage to flower buds (squares), flowers and young fruit (bolls) does not affect marketable yield. Due to the indeterminate nature of cotton, production of squares, flowers and bolls continues throughout much of the latter part of the growing season. However, after a certain point in the growing season there is insufficient time for squares, flowers and young bolls to mature and contribute to marketable yield with or without further insecticide applications. Because mature and near-mature bolls are not particularly susceptible to damage by the major insect pests of cotton, stopping insecticide applications later in the season might not significantly reduce yield. Although reducing late-season insecticide applications may make economic sense in the short term, the lack of late-season control one season will most likely lead to larger insect populations earlier in the next season.
Some key insect pests of cotton include the bollworm (Helicoverpa zea), tobacco budworm (Heliothis virescens) and boll weevil (Anthonomus grandus). In the case of bollworms and budworms, moths lay eggs usually in the terminal area (shoot apex) of the cotton plant. After hatching, young larvae feed on the terminals and younger squares, and larger larvae feed on terminals and young bolls. For the boll weevil, adult females eat a cavity into a square and lay a single egg. Upon hatching, the larvae feeds inside the square and undergoes two or three molts. Squares containing boll weevil larvae usually abscise from the plant and fall to the ground a few days after the development of the second stage larvae. Boll weevil development continues in the abscised square.
A crop management strategy that reduces or eliminates the feeding and/or oviposition sites of insect pests might eliminate the need for late-season insecticide applications and also lower the populations of overwintering insects. In the case of the boll weevil, a crop management strategy that reduces late season squares should reduce the overwintering boll weevil populations because it is known that boll weevils that enter diapause later in the season are more likely to overwinter successfully than boll weevils that enter diapause earlier.
It is towards providing a successful early-termination strategy which meets, or exceeds, the goals noted above that the present invention is directed. Broadly, the present invention is directed toward the late-season termination of reproductive plant growth of a plant having an indeterminate growth pattern. More particularly, according to the present invention, a semicarbazone plant growth regulator is applied to a locus of a growing plant at a time following peak bloom of the plant in an amount sufficient to terminate continued reproductive growth of the plant without substantially affecting mature reproductive growth existing on the plant at the time of application. Most preferably, the semicarbazone is diflufenzopyr. By using such an early growth-termination strategy, improvements in the plant's late-season fruit yield as well as improved late-season pest management may be achieved.
These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.